1. Field of the Invention
The invention relates to an equalizer, and more particularly to an equalizer with auto-calibration and self-test.
2. Description of the Prior Art
As generally know in the art of data communications systems, when a transmission medium such as a transmitter sends analog or digital signals over a transmission medium such as a transmission line or cable to a receiver, the received signals may become distorted due to attenuation and phase delay resulting when the signals are transmitted through the transmission line.
For example, a 100BaseT Ethernet receiver (following the Ethernet standard ANSI/IEEE Std. 802.3u) at the rate of 125 megabits per second. However, bit error rate for such as Ethernet receiver is related to signal distortion introduced to a received signal by a transmission line, and such distortion will vary depending on the length of transmission line utilized. A signal can be provided to an Ethernet receiver which follows the Ethernet standard ANSI/IEEE Standard 802.3u through a transmission line which may have one of several different lengths. In order to compensate for the signal distortion introduced by a transmission line, signals are typically corrected before being provided to an Ethernet receiver by passing the signals through an equalizer circuit which makes corrections to the input signal for amplitude losses and phase delay introduced by a transmission line.
A typical equalizer circuits include a compensating filter which restores amplitude and phase delay to a signal caused by a cable by having a gain and phase shift controlled by a received control signal. The control signal is typically provided by circuitry utilizing feedback form the equalizer.
A control circuit using feedback from an equalizer uses a set of high-pass basis functions that may be weighted and summed. By adjusting the weights, the compensating circuit can be adapted to accurately control the equalizer to compensate for distortion caused by various cable lengths. These fixed weighting terms for a particular cable length are typically set by a ratio of resistances coupled to the equalizer output. The structure assumes that the variations of the RC time constant controlled by varying the resistances will be roughly equivalent to a shift in RC time delay resulting from a cable length, and the assumption will apply as long as the ratios of resistances remain fairly constant over process. Although this technique is satisfactory for some cable lengths, the weights are difficult to control to accurately compensate for all the different cable length.
Referring to FIG. 1, a conventional equalizer 100 comprises an automatic gain control (AGC) circuit 130, an analog to digital (A/D) converter 140, digital adaptive equalizer 170(equalizer core), timing recovery 150 and an automatic gain control loop 160. The intensity of a received signal Vs is adjusted by the AGC circuit 130 and then converted a received digital signal by the A/D converter 140. The digital adaptive equalizer 170 adjusts the phase of the received digital signal to a received equalized signal Ve. The AGC circuit 130 and the A/D converter 140, coordinated with the AGC loop 160 and the timing recovery 150, are for use of intensity adjustment of the various received signals, while the digital adaptive equalizer 170 is for use of distortion compensations for the various frequencies. Such a conventional equalizer adjusts the gain and phase of the received signal at different time.
However, existing equalizer needs one ADC and digital equalizer. Additionally, the adjustments of the gain and phase for the received signal are implemented at two different stages.
It is an object of the present invention to provide an initialization circuit with continuous feedback equalizer. A feedback control circuit provides continuous auto-calibration for an equalizer core circuit.
It is another object of the present invention to provide a transmission line length estimation circuit capable to implement auto-calibration and self-test. The self-test steps with a pattern generator circuit can reduce problem of the calibration difficulty.
In the present invention, a transmission line length estimation circuit with continuous feedback equalizer is provided. The transmission line length estimation circuit comprises a DC bias current and peak detector circuit to generate a DC voltage according to different cable length. An equalizer core circuit is for receiving a first signal and generating a second signal. The peak detector circuit is used for the first signal and the second signal in the different time. A transmission line length detector circuit is coupled to the peak detector circuit and used for generating a plurality of first parameters for phase shift and amplitude losses according to different cable length. An internal pattern calibration circuit is multiplexed to the first signal and used for generating a plurality of second parameters for calibration of close loop. A feedback control circuit is connected to the equalizer core circuit and used for continuously fine tuning of the equalizer core circuit according to the first parameters and the second parameters